In an engine, such as an automobile engine, it is well-known to have a variable valve mechanism that varies an opening-closing timing of an intake valve and an exhaust valve, and controls a variable valve mechanism so as to make an overlap amount in the open states of an intake valve and of an exhaust valve large when the engine speed is high, and make it small when the engine speed is low in order to improve the engine performance.
When the variable valve mechanism is broken, it may cause a decrease of the fuel efficiency and the deterioration of the exhaust emission as well as the decrease of the engine performance, so that the engine having such a variable valve mechanism needs to diagnose the abnormality of the variable valve mechanism.
Correspondingly, as for the abnormality diagnosis of the variable valve mechanism, for example, the Patent Document 1 disclosed that an abnormality diagnosis of the variable valve mechanism, which changes the overlap amount in the open state of the intake valve and the open state of the exhaust valve by switching the opening-closing time of the intake valve, is performed based on the difference between the actually measured intake air amount and the predicted intake air amount based on a throttle valve opening degree and a rotational speed.